You can actually getting GPUs from the Chinese markets (e.g., AliExpress) that have had their VRAM upgraded. Someone out there is doing aftermarket VRAM upgrades on cards to make them more usable for GPGPU tasks.
Which also answers your question: The manufacturers aren't doing it because they're assholes.
These are a bit mythical, finding one for sale is no small feat.
I guess adding memory to some cards is a matter of completely reworking the PCB, not just swapping DRAM chips. From what I can find it has been done, both chip swaps and PCB reworks, it's just not easy to buy.
Software support is of course another consideration.
Which also answers your question: The manufacturers aren't doing it because they're assholes.